Documented layout source-path $SNAP_USER_DATA is not actually allowed

This document (https://snapcraft.io/docs/snap-layouts) says:

<source-path> must refer to either $SNAP, $SNAP_DATA or $SNAP_USER_DATA.

However, snapcraft ver. 4.5.4.post11+git82024d37 shows the following error.

layout “/foo/bar” uses invalid symlink old name “/tmp”: must start with $SNAP, $SNAP_DATA or $SNAP_COMMON

Alternatively, the error will be the following.

layout “/foo/bar” uses invalid symlink old name “$SNAP_USER_DATA/tmp”: reference to unknown variable “$SNAP_USER_DATA”

Which is the correct behaviour? I assume the document is wrong.